The present invention relates to a system for transferring heat energy within a gas turbine engine, and more particularly to a system which provides an actuator pump to minimize the usage of heat exchangers.
The fluid flow requirements of gas turbine engines are well known to the designers of today""s high performance aircraft powerplants. Certain internal structures, such as bearings, are both cooled and lubricated by a circulating flow of oil which is distributed and collected throughout the main engine structure. Another thermal management method includes rejecting heat from the circulating oil loops into the flow of fuel entering the engine combustion chamber. This method uses the fuel flow as a recuperative heat sink which incurs few of the penalties of air cooling, but is limited in effectiveness by the maximum temperature tolerable by the fuel. Further effectiveness of using the flow of fuel, is the limitation necessitated by maintaining the fuel flow above freezing to minimize the possibility of ice formation and subsequent entry into sensitive areas such as engine actuators.
Main fuel pumps for aircraft gas turbines have traditionally been fixed delivery, positive displacement type pumps connected mechanically to the rotating engine shaft. As the flow rate from a pump turning proportional to engine shaft speed cannot match the fuel flow requirements of a gas turbine engine operating under a variety of power levels, it is common to size the main fuel pump with an excess flow capacity under all engine operating conditions. The fuel system therefore include a fuel bypass for routing excess main fuel flow back to the low pressure side of the main pump. Such fluid flow system requirements results in complex thermal management requirements.
Cooling oil circulating through the main engine lubrication system receives heat energy at a rate related to the product of engine rotor speed and power output. The cooling needs of the main engine lubrication loop are thus at a minimum during periods of low power operation, such as idling, and at a maximum during high or full power operation, such as takeoff.
Under certain operating conditions, such as engine idling, the amount of fresh fuel entering the fuel system is small while the relative volume of fuel being bypassed back to the pump inlet is quite large. The combination of pump inefficiency and recirculation of excess fuel through the fuel bypass may heat the circulating fuel to an undesirably high temperature. This excess heat must be rejected to assure that the fuel remains within its maximum tolerable temperature.
Excess heat is commonly managed by a combination of fuel/oil and air/oil heat exchangers. Such heat exchangers are undesirable because of their negative impact on engine efficiency, weight and expense. Thermal energy rejected from the engine oil does not contribute to engine thrust, while the majority of thermal energy rejected from the oil to the fuel is recovered at the engine burner stage. Additionally, cooling air for the air/oil coolers is typically bled from a cool high-pressure air source such as engine fan discharge which further reduces engine thrust.
Conversely, other operating conditions result in insufficient fuel heating which may also become a concern. The fluid flow system must provide sufficient fuel heating to prevent entrained water from freezing and possibly blocking small openings in the fuel system actuator servos. For example, at certain high power conditions such as cold day takeoff, the heat available from the circulating lubricating oil is often insufficient to heat the high volume of relatively cold fuel to a temperature above freezing. A servo heater is commonly provided to assure the high pressure flow which operates the actuators will not freeze. Further negative impact on engine efficiency, weight and expense thus results.
Accordingly, it is desirable to provide a fluid flow system for a gas turbine engine which significantly reduces heat generation at low flow demand to minimize the size and number of heat exchangers. It is further desirable to regulate actuator flow temperature at high fluid flows to preclude freezing of water entrained in the fuel.
The fluid flow system for a gas turbine engine according to the present invention provides combustion fuel to a main pump and an actuator pump. From the actuator pump, fuel is communicated through an actuator junction where high pressure fuel is supplied to an actuator minimum pressure valve (AMPV) and into a Thermal Bypass Valve (TBV.) The actuator junction includes a filter to further filter the high pressure fuel prior to entering an engine actuator. Engine actuators are high pressure fluid actuators which operate engine components such as inlet guide vanes, bleed valves, turbine cooling valves, nozzle actuators and the like.
The actuator pump is preferably sized to provide actuator steady state plus transient flow to assure positive operation of the actuators. The AMPV regulates actuator pump discharge pressure above actuator pump inlet pressure to the minimum pressure required to assure the positive operation of the actuators. That is, the AMPV assures that the actuator flow pressure provided by the actuator pump is that which is required to operate the actuator.
Fuel flow from the actuator pump in excess of the actuators needs is directed through the AMPV and into the TBV. Depending upon the temperature of the fuel, the TBV determines the path of the excess actuator pump fluid flow. The TBV divides the fuel flow between being recirculated to the actuator pump inlet and the main pump output flow path to the engine fuel input conduit. Preferably, when the fuel is near the freezing point of water, the TBV will recirculate the fuel to the actuator pump inlet to raise the fuel temperature within the actuator fuel flow circuit without the heretofore-required servo heater. The engine actuators are thereby assured of receiving flow which precludes freezing of water entrained in the fuel. When there is minimal concern with the possibility of freezing water entrained in the fuel, the TBV passes a greater percentage of fuel through to join together in the engine fuel input conduit. The AMPV assures that the actuator flow pressure is always that which is required to operate the actuators.
The present invention therefore provides a fluid flow system for a gas turbine engine which significantly reduces heat generation at low flow demand to minimize the demand for heat exchangers while regulating actuator flow temperature at high fluid flows to preclude freezing of water entrained in the fuel.